Process for deforming a piece of thin-walled metal tube

ABSTRACT

Process for deforming a piece of thin-walled metal tube, comprising the operations of bending the piece of tube with respect to its original axis, in which process the wall of the piece of tube is firstly provided, at least at the location where the bend is intended to be, with a deformation which extends substantially in the longitudinal direction of the piece of tube, and in which process wall material is moved closer to the neutral plane of bending, and the piece of tube is then bent, after which the ultimate shape of the piece of tube which is to be deformed is imposed by hydroforming.

The invention relates to a process for deforming a piece of thin-walledmetal tube, comprising the steps of bending the piece of tube withrespect to its original longitudinal axis and then hydroforming thepiece of tube at least at the location where it has been bent in thisway.

A known method for deforming pieces of tube comprises what is known ashydroforming. In this process, the wall of the piece of tube is pressedagainst a mould piece under the influence of water pressure, so that thepiece of tube acquires its ultimate shape. The hydroforming technique isgenerally known and therefore does not require any further explanationhere. If the piece of tube is also to be bent, a bending operation iscarried out prior to and separately from the hydroforming step, in whichcase the bent piece of tube then acquires its ultimate, desired shapethrough hydroforming. In this way, numerous very complicated shapes canbe produced, which are used in engineering, for example in theautomotive industry.

It has been found that, in this processing method, the bending of thepiece of tube forms a critical step. Particularly if a small wallthickness is used for the piece of tube, cracks are rapidly formed alongthe outside of the bend during the bending operation, while wrinkles areformed in the compressed region along the inside of the bend. Thesewrinkles in this case occur in the circumferential direction of thepiece of tube, i.e. they run in a direction which is transverse withrespect to the longitudinal axis of the piece of tube. If the bent pieceof tube is then subjected to a hydroforming operation, it is found thatwrinkles of this nature running in the circumferential direction can nolonger be removed. The result is an unacceptable product.

JP-61-086029-A discloses a process for deforming a metal tube of theabove mentioned kind which involves a pressure-crushing step prior tothe bending of the tube. In this pressure-crushing step, the tube isfitted into a hole of a crushing device, which has the same diameter asthe outer diameter of the tube, and then a punch is driven against thetube wall at the location, where the tube is to be bent in the laterbending step. Thereby the section of the tube, which comes into contactwith the punch, is pressure-crushed and brought into close contact withthe pipe wall on the opposite side. After this pressure-crushing stepthe tube is inserted into a hole of a tube bending device such that thestraight tube wall part, to which the pressure-crushed section of thetube wall of the tube has been brought into contact, is positioned so asto abut a crest-form apex part of the lower edge surface of the hole.Then a punch is driven through a hole of the die against thepressure-crushed section of the tube wall whereby the tube is bent alongthe crest-form apex part.

In this prior art process generation of creases is said to be avoided inthe bending step, since the wall sections of the tube abutting thecrest-form apex part are pinched-pressured from both sides during thebending process. However, in the pressure-crushing step the wall sectionhas to be deformed to a high extent in order to bring it into closecontact with the wall section on the opposite side. Accordingly thisprocess is hardly applicable to thin-walled tubes.

Therefore, the object of the invention is to provide a method in whichthe production of bent, thin-walled pieces of tube and of productsformed using such pieces of tube causes fewer problems. In particular,it is intended to reduce the risk of cracking and to avoid the formationof wrinkles in the circumferential direction.

In the process described in the preamble, this is achieved if the pieceof tube, before it is bent, is indented on both the inside and theoutside of the bend which is to be formed at the location where it willexhibit a bend as a result of the bending operation.

As a result of the indentation, which involves moving wall materialcloser to the neutral plane of bending stresses, the piece of tube canbe bent more easily, with the result that the risk of cracking and theformation of wrinkles in the circumferential direction is reducedconsiderably. The ultimate shape of the piece of tube which is to bedeformed can then be imposed during the subsequent hydroformingoperation, during which any deformations which may have been imposed inthe longitudinal direction can be eliminated, or further deformationscan be produced. It should be noted that, surprisingly, it has beenfound that wrinkles in the wall of the bent piece of tube which run inthe longitudinal direction, i.e. more or less parallel to the axis ofthe piece of tube, are no longer visible after the hydroforming hastaken place.

The process can be used in order to impart a constant cross section tothe deformed piece of tube along its length, in that the undeformedpiece of tube is firstly provided with longitudinal wrinkles on eitherside of the neutral plane as a result of compression, and then, afterthe bending operation, the constant cross section is restored by thehydroforming step. It has been found that in this way pieces ofthin-walled tube with a constant cross section and a relatively smallradius of curvature can be produced successfully. In the past, theproduction of pieces of tube of this nature caused considerableproblems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a straight piece of tube;

FIG. 1b shows the piece of tube of FIG. 1a after the tube's wall hasbeen compressed from either side half-way along the piece of tube;

FIG. 1c shows the piece of tube of FIG. 1b bent;

FIG. 1d shows a bend piece made from hydroforming the tube of FIG. 1c;

FIG. 1e shows the cross-section along view Ie—Ie of FIG. 1b

FIG. 1f shows the cross-section along view If—If of FIG. 1b;

FIG. 1g shows the cross-section along view Ig—Ig of FIG. 1c;

FIG. 2a ₁ a front view of a piece of tube;

FIG. 2a ₂ shows a cross section along view IIa₂—IIa₂ through the pieceof the tube of FIG. 2a ₁.

FIG. 2b ₁ a front view of the piece of tube of FIG. 2a ₁ wherein atlocation 7, longitudinal wrinkles are pressed into the tube at location7;

FIG. 2b ₂ shows a cross section along view IIb₂—IIb₂ through a piece ofthe tube of FIG. 2b ₁.

FIG. 2c ₁ shows a front view of a shape obtained through bending of thepiece of tube of FIG. 2b ₂.

FIG. 2c ₂ shows a cross section view along view IIc₂—IIc₂ through thepiece of the tube of FIG. 2c ₂.

FIG. 2d ₁ shows a side view of the product obtained by hydroforming thebent tube of FIG. 2c ₁.

FIG. 2d ₂ shows a cross section along view IId₂—IId₂ through the productof FIG. 2d ₁.

DESCRIPTION OF PREFERRED EMBODIMENTS

In one embodiment of the process according to the invention, the pieceof tube, after the indenting step and before and/or after the bendingstep, is deformed in such a manner that at least part of the materialwhich forms the piece of tube, which part, as seen in cross sectionthrough the piece of tube, is situated in a region between regions wherethe piece of tube is indented, is moved closer to the centre of gravityof the cross section. This further reduces the risk of cracking and/orwrinkling.

The invention will now be explained with reference to a number offigures.

FIGS. 1a-d show the production of a bend piece of constant cross sectionfrom a straight piece of tube.

FIG. 2 shows the production of a tubular product of complicated shapefrom a straight piece of tube.

In FIGS. 1, a, b, c and d illustrate various phases of the production ofthe bend piece 4. In

FIG. 1a, reference numeral 1 denotes a straight piece of tube which isto be formed into the bend piece 4 shown in FIG. 1d.

FIG. 1b shows the piece of tube 1 with cross section 2 after the wallhas been compressed from either side half-way along the piece of tube,with the result that wall wrinkles are formed in the longitudinaldirection. The cross section of the piece of tube at the location ofthese longitudinal wrinkles is indicated by 3. The arrows which aredirected towards one another diagrammatically indicate the indentationaccording to the invention. Arrows directed towards one another at 3′indicate any further deformation of the piece of tube before or afterbending FIG. 1c shows how the piece of tube can easily be bent at thelocation of the thinned middle piece, partly as a result of theconsiderable reduction in the section modulus of the cross section atthat location. The risk of cracks or wrinkles in the circumferentialdirection occurring is considerably reduced as a result. The shape shownin FIG. 1d can be obtained by subjecting the product shown in FIG. 1c toa hydroforming operation.

FIG. 2 illustrates the production of a workpiece with a more complicatedshape. In FIGS. 2a 1 and a 2, a front view of and a cross sectionthrough a piece of tube are illustrated. FIGS. 2d 1 and d 2 show a sideview of and a cross section through the product 9 formed therefrom. Atlocation 7, longitudinal wrinkles are pressed into the piece of tube 5,with the result that a cross section of the shape of 8 is formed at thatlocation (cf. FIGS. 2b 1 and b 2). The shape shown in FIGS. 2c 1 and c 2is obtained through bending of the piece of tube.

From this, the final shape 9 can be obtained by hydroforming; numerousvariations on this shape are conceivable.

What is claimed is:
 1. A process for deforming a piece of a thin-walledmetal tube, comprising the steps of: bending the piece of tube withrespect to an original longitudinal axis of the piece of tube, and thenhydroforming the piece of tube at least at the location where the pieceof tube has been bent, compressing the piece of tube prior to thebending step at the location where the piece of tube will exhibit thebend as a result of the bending step so as to form a wrinkle comprisinga longitudinal indentation, which runs substantially parallel to thelongitudinal axis of the tube on the outside of the bend to be formed,wherein in the compression step prior to the bending step at least onesaid longitudinal wrinkle is formed at the outside of the bend and atleast one said longitudinal wrinkle is formed at the inside of the bendon either side of a neutral plane of bending stresses of the bend,thereby moving wall material on both sides of the neutral plane closerto the neutral plane, wherein the tube prior to bending defines atransverse perimeter of a cross-section of the tube, the transverseperimeter is transverse to the longitudinal axis of the tube and passesthrough the longitudinal wrinkles, wherein the transverse perimeter hasat least one indented portion formed respectively by said at least onewrinkle formed at the outside of the bend and at least one indentedportion formed respectively by said at least one wrinkle formed at theinside of the bend, such that at least one indented portion is locatedon either side of the neutral plane of bending stresses of the bend,wherein prior to said bending some points of said transverse perimeterare further from the longitudinal axis of the tube than other points ofthe transverse perimeter, wherein the neutral plane passesperpendicularly through the transverse cross section prior to thebending; wherein the transverse perimeter after forming the longitudinalwrinkles, but prior to bending, includes portions which overlap a secondtransverse perimeter of a portion of the tube which is not compressed.2. The process of claim 1, wherein the transverse perimeter at alongitudinal midpoint of at least one said longitudinal indentation,prior to bending, includes said portions which overlap said secondtransverse perimeter of said portion of the tube which is notcompressed.
 3. The process of claim 1, wherein after said compressingbut prior to said bending, the length of the transverse perimeter of thetube is constant along the entire length of the tube.
 4. The process ofclaim 1, wherein after said compressing but prior to bending, the lengthof the transverse perimeter of the tube including the indented portionsequals the length of a second transverse perimeter of a section of thetube which is not compressed.
 5. The process of claim 1, wherein eachsaid indented portion does not touch any other indented portion.
 6. Theprocess of claim 1, wherein each said indented portion does not touchany other indented portion.
 7. The process of claim 1, consisting ofsaid bending, hydroforming and compressing steps.
 8. A process fordeforming a piece of a thin-walled metal tube, comprising the steps of:bending the piece of tube with respect to an original longitudinal axisof the piece of tube, and then hydroforming the piece of tube at leastat the location where the piece of tube has been bent, compressing thepiece of tube prior to the bending step at the location where the pieceof tube will exhibit the bend as a result of the bending step so as toform a wrinkle comprising a longitudinal indentation, which runssubstantially parallel to the longitudinal axis of the tube on theoutside of the bend to be formed, wherein in the compression step priorto the bending step at least one said longitudinal wrinkle is formed atthe outside of the bend and at least one said longitudinal wrinkle isformed at the inside of the bend on either side of a neutral plane ofbending stresses of the bend, thereby moving wall material on both sidesof the neutral plane closer to the neutral plane, wherein the tube priorto bending defines a transverse perimeter of a cross-section of thetube, the transverse perimeter is transverse to the longitudinal axis ofthe tube and passes through the longitudinal wrinkles, wherein thetransverse perimeter has at least one indented portion formedrespectively by said at least one wrinkle formed at the outside of thebend and at least one indented portion formed respectively by said atleast one wrinkle formed at the inside of the bend, such that at leastone indented portion is located on either side of the neutral plane ofbending stresses of the bend, wherein prior to said bending some pointsof said transverse perimeter are further from the longitudinal axis ofthe tube than other points of the transverse perimeter, wherein theneutral plane passes perpendicularly through the transverse crosssection prior to the bending; wherein the tube following hydroforminghas a constant transverse cross-section along its entire length.
 9. Theprocess of claim 8, consisting of said bending, hydroforming andcompressing steps.
 10. A process for deforming a piece of a thin-walledmetal tube, comprising the steps of: bending the piece of tube withrespect to an original longitudinal axis of the piece of tube, and thenhydroforming the piece of tube at least at the location where the pieceof tube has been bent, compressing the piece of tube prior to thebending step at the location where the piece of tube will exhibit thebend as a result of the bending step so as to form a wrinkle comprisinga longitudinal indentation, which runs substantially parallel to thelongitudinal axis of the tube on the outside of the bend to be formed,wherein in the compression step prior to the bending step at least onesaid longitudinal wrinkle is formed at the outside of the bend and atleast one said longitudinal wrinkle is formed at the inside of the bendon either side of a neutral plane of bending stresses of the bend,thereby moving wall material on both sides of the neutral plane closerto the neutral plane, wherein the tube prior to bending defines atransverse perimeter of a cross-section of the tube, the transverseperimeter is transverse to the longitudinal axis of the tube and passesthrough the longitudinal wrinkles, wherein the transverse perimeter hasat least one indented portion formed respectively by said at least onewrinkle formed at the outside of the bend and at least one indentedportion formed respectively by said at least one wrinkle formed at theinside of the bend, such that at least one indented portion is locatedon either side of the neutral plane of bending stresses of the bend,wherein prior to said bending some points of said transverse perimeterare further from the longitudinal axis of the tube than other points ofthe transverse perimeter, wherein the neutral plane passesperpendicularly through the transverse cross section prior to thebending; wherein the transverse perimeter of the cross-section of thetube prior to bending defines a five pointed star.
 11. The process ofclaim 10, consisting of said bending, hydroforming and compressingsteps.